Sintered alumina product that is transparent to infrared radiation

Abstract

A sintered alumina product includes, as a percentage by weight, more than 99.95% of alumina (Al 2 O 3 ), the grain size of the alumina being in the range 0.2 μm to 1.5 μm, and has a density greater than 99.95% of the theoretical density of the alumina.

1 - 14 . (canceled) 
   
   
       15 . A sintered alumina product comprising, as a percentage by weight, more than 99.9S% of alumina (Al 2 O 3 ), the grain size of the alumina being in the range 0.2 μm to 1.5 μm, and having a density greater than 99.95% of the theoretical density of the alumina. 
   
   
       16 . A product according to  claim 15 , wherein the grain size of the alumina is over 0.3 μm. 
   
   
       17 . A product according to  claim 15 , wherein the grain size of the alumina is less than 1.0 μm. 
   
   
       18 . A product according to  claim 15 , wherein the grain size of the alumina is over 0.3 μm and less than 1.0 μm. 
   
   
       19 . A product according to  claim 15 , wherein the grain size of the alumina is over 0.45 μm. 
   
   
       20 . A product according to  claim 15 , wherein the grain size of the alumina is less than 0.75 μm. 
   
   
       21 . A product according to  claim 15 , wherein the grain size of the alumina is over 0.45 μm and less than 0.75 μm. 
   
   
       22 . A product according to  claim 15 , comprising a surface density (Fv) of grains with a diameter greater than twice the mean diameter of the other grains of less than 4% of the surface area. 
   
   
       23 . A product according to  claim 18 , comprising a surface density (Fv) of grains with a diameter greater than twice the mean diameter of the other grains of less than 4% of the surface area. 
   
   
       24 . A product according to  claim 21 , comprising a surface density (Fv) of grains with a diameter greater than twice the mean diameter of the other grains of less than 4% of the surface area. 
   
   
       25 . A product according to  claim 22 , containing no grains with a diameter greater than twice the mean diameter of the other grains. 
   
   
       26 . A product according to  claim 23 , containing no grains with a diameter greater than twice the mean diameter of the other grains. 
   
   
       27 . A product according to  claim 24 , containing no grains with a diameter greater than twice the mean diameter of the other grains. 
   
   
       28 . A product according to  claim 15 , having a three-point bending strength at 20° C. greater than 650 MPa. 
   
   
       29 . A product according to  claim 15 , having a real in-line transmittance (RIT) greater than 80% for wavelengths of incident radiation in the range 2.5 μm to 4.5 μm. 
   
   
       30 . A method of fabricating a sintered alumina product comprising the following steps in succession:
 a) preparing a slip from an alumina powder with an elementary particle size in the range 0.02 μm to 0.5 μm;   b) casting the slip into a porous mold then drying and unmolding to obtain a green part;   c) drying the unmolded green part;   d) debinding at a temperature in the range 350° C. to 500° C.;   e) sintering at a temperature in the range 1100° C. to 1350° C. until a sintered product is obtained with a density of at least 92% of the theoretical density of the alumina; and   f) carrying out hot isostatic pressing, “HIP”, at a temperature in the range 950° C. to 1300° C. at a pressure in the range 1000 to 3000 bars.   
   
   
       31 . A method of fabricating a sintered alumina product according to  claim 30 , in which the mold is dried prior to casting the slip. 
   
   
       32 . A method of fabricating a sintered alumina product according to  claim 30 , in which the temperature is in the range 20° C. to 25° C. during the whole of step b). 
   
   
       33 . A method of fabricating a sintered alumina product according to  claim 30 , in which the pressure of the slip inside the mold is in the range 1 bar to 1.5 bar. 
   
   
       34 . A method of fabricating a sintered alumina product according to  claim 30 , in which the moisture content of the environment of the mold is maintained at between 45% and 55% for the whole of step b). 
   
   
       35 . A method of fabricating a sintered alumina product according to  claim 30 , in which the hot isostatic pressing is carried out at a temperature below the sintering temperature. 
   
   
       36 . A method according to the  claim 35 , in which the hot isostatic pressing temperature is 20° C. to 100° C. lower than the sintering temperature. 
   
   
       37 . A furnace observation window or a missile dome comprising the sintered alumina product of  claim 15 . 
   
   
       38 . A method of fabricating a sintered alumina product according to  claim 30 , wherein, in step a), dispersion of the alumina powder grains in the slip is improved using beads. 
   
   
       39 . A method of fabricating a sintered alumina product according to  claim 38 , wherein the amount of alumina in said beads is over 99.5% by volume.